New relationships are identified between the composition and texture of clastic rocks and their Coulomb failure parameters, unconfined compressive strength and friction angle. Rocks examined in this study have porosities that vary from 0% to 40% and the volume fraction of clay minerals varies from 0% to 72%. Results show that unconfined compressive strength depends on porosity, clay content and the degree of cementation. Young's modulus was found to be a better indicator of unconfined compressive strength than porosity. Friction angle depends on porosity, clay content and effective confining pressure. These results indicate that measurements of porosity, clay content, cementation and in-situ stress are required to evaluate strength variations among rocks typically encountered in a single well.